The invention relates to a broad-band hydraulic vibrator for use as a seismic source and having an adjustable piston for reducing cylinder volume at high frequencies.
Hydraulic vibrators used as a seismic source generate a spectrum of frequencies ranging from a low frequency to a high frequency, to produce a frequency sweep over the desired range of operation. During low frequency operation, the piston stroke of the vibrator is relatively long and encompasses a large cylinder volume. In contrast, at higher frequency operation, the piston stroke of the vibrator is shortened and encompasses a smaller cylinder volume.
The large volume of hydraulic fluid within the cylinder which is necessary for low frequency operation has a compressibility factor which impairs efficient vibrator operation during high frequency operation. Therefore, mechanisms have been developed to provide variable cylinder volumes in the hydraulic vibrator to optimize the operation of the vibrator at different frequencies of operation. The adjustment mechanisms utilize movable end sleeves or cylinder heads which are positioned at opposite ends of the cylinder. The mechanisms are positioned in an extended position during low frequency, long-stroke operation and are moved inward to reduce cylinder volume during high frequency, short-stroke operation.
The adjustment mechanisms can be operated manually by setting the position of the mechanism for a frequency sweep or under an automatic control which positions the adjustment mechanism depending on the vibrator frequency during the frequency sweep. Another control system provides a dual-mode operation with a set frequency to change the adjustment from long-stroke position to short-stroke position when the sweep frequency exceeds the set frequency.